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Silent Synapses in Neural Plasticity: Current Evidence

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Silent Synapses in Neural Plasticity: Current Evidence Downloaded from learnmem.cshlp.org on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press REVIEW Harold L. Atwood1 and 1 Silent Synapses in Neural Plasticity: J. Martin Wojtowicz Current Evidence Department of Physiology Medical Sciences Building University of Toronto Toronto, Ontario M5S 1A8, Canada Abstract Silent synapses, defined as structural specializations for neurotransmission that do not produce a physiological response in the receiving cell, may occur frequently in neural circuits. Their recruitment to physiological effectiveness may be an important component of circuit modification. In several nervous systems, evidence from electrophysiological and optophysiological measurements has established a strong case for the existence of silent synapses and for their emergence as active synapses with appropriate stimulation. During normal development and aging, synapses of individual neurons change in number, and many of these may be functionally silent at certain stages of their developmental trajectory. Changes in their status may contribute to shaping the properties of neural pathways during development, often in response to neural activity. In general, it is often difficult to distinguish physiological emergence of pre-established silent synapses from developmental maturation or de novo formation of new synapses. Several possible mechanisms for silent synapses and their recruitment are reviewed. These include incompletely assembled synapses that lack structural components, insufficient availability of key presynaptic proteins, and nonfunctional postsynaptic receptors, or presence of receptors that do not mediate a postsynaptic response except under specific conditions (conditionally silent synapses). The available silent synapses can often be rapidly activated, and conversely, active synapses appear to be rapidly silenced in many instances. These properties enable silent synapses to participate in short-term facilitation and depression. In addition, they may contribute to long-term facilitation and potentiation, especially during development. Introduction POTENTIAL SIGNIFICANCE The mammalian central nervous system (CNS) contains many millions of OF SILENT SYNAPSES neurons, each of which characteristically receives thousands of individual inputs from other neurons and in turn provides thousands of outputs to other neurons. The sites of input and output are the synapses, which can be structurally defined with electron microscopy. Individual synapses vary enormously in their biochemical properties and physiological effects. The transmitter substances emitted by neurons at synapses, the receptors 1Corresponding authors. LEARNING & MEMORY 6:542–571 © 1999 by Cold Spring Harbor Laboratory Press ISSN1072-0502/99 $5.00 LEARNING& MEMORY 542 Downloaded from learnmem.cshlp.org on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press SILENT SYNAPSES activated by the neurotransmitters, and the ensuing physiological responses (excitatory, inhibitory; facilitating, depressing; strong, weak; ionotropic, metabotropic) are all tremendously variable, and susceptible to modification by neural activity or during development. A major question about neural synapses is as follows: Are all of the structurally defined synapses visible in electron micrographs physiologically potent? Since the early 1970’s, more and more evidence has accumulated that suggests that they are not. In particular, two early observations pointed to the occurrence of ineffective or incomplete (and thus, potentially silent) synapses. First, ultrastructural studies of crustacean motor neurons showed many synapses that appeared to lack docked (releasable) synaptic vesicles, and there were many more synapses than quanta released during neural activity (Jahromi and Atwood 1974). Secondly, and more dramatically, in the mammalian CNS, circumstances were found in which receptive fields could be suddenly expanded, implying the existence of newly awakened (previously silent) synapses (Wall 1977). Such observations indicated a strong possibility that many of the individual synapses formed between pairs of neurons are effectively silent, and do not produce a physiological effect when the presynaptic neuron is activated. It could even be that a majority of synapses in many neural pathways are physiologically silent, but such synapses might be recruited to physiological effectiveness with specific patterns of neural activity or through actions of neuromodulators and hormones. If so, this type of recruitment could be a major mechanism in pathway consolidation, learning, and memory. One can imagine that in many parts of the nervous system, there is a large reserve of silent synapses, which can be jolted into an active state by the right combination of stimuli, and can then enhance transmission either transiently or permanently in the neural circuits to which they contribute. According to this view, the nervous system is normally operating well below capacity, but has the potential for great enhancement and reconfiguration of local circuits. Silent synapses provide one mechanism (perhaps a major one) for enhanced performance of the nervous system. The potential importance of silent synapses is a stimulus to review evidence supporting their existence and their possible functional significance. Evidence for silent synapses, particularly in the mammalian CNS, has been reviewed in several recent articles (Malenka and Nicoll 1997, 1999; Malenka 1998; Malinow 1998). We wish to extend the scope of the discussion to include other well-studied examples that offer relevant evidence and additional opportunities. New examples and evidence are appearing at an increasing rate. First, we examine some basic features of synapses that might be involved in creation of silent synapses; then, we examine a selection of currently available evidence; and finally, we review briefly possible mechanisms and functional significance. SYNAPSE Synapses have both structural and functional attributes that are interrelated. Electron microscopy permits resolution of structural specialization at points of contact between nerve cells and their postsynaptic targets. The structural specialization is thought to reflect molecular features that impart the capability for fast release of neurotransmitter (fast exocytosis) from the presynaptic neuron during an LEARNING& MEMORY 543 Downloaded from learnmem.cshlp.org on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press Atwood and Wojtowicz action potential. Most commonly, the structural definition of a synapse (Korn 1998) is taken to be the individual contact, comprising electron-dense pre- and postsynaptic membranes aligned rigidly with uniform (20–50 nm) separation (the synaptic cleft), and including variable specialized structures in both pre- and postsynaptic compartments. As illustrated in Figure 1, A and C, synaptic terminals or their varicosities often provide many individual synapses to a postsynaptic target cell, but in many cases, only a single synapse is present. In the literature, other uses of the term synapse appear; these have been reviewed recently by Korn (1998). The totality of synapses (which may be many) between one neuron and another, or between a neuron and a non-neural target, is commonly termed a synaptic connection (for review, see Faber et al. 1991). Associated with the individual synapse’s presynaptic membrane, dense projections often occur. Although their composition is not fully known, they appear to be focal points for accumulations of synaptic vesicles and are thus likely involved in directed docking and tethering of these vesicles, which are preferentially released at the margins of the presynaptic specializations in arthropod neuromuscular junctions (Fig. 1F). The presynaptic subregion involved in preferential release of neurotransmitter from synaptic vesicles is generally termed the active zone, from studies on the neuromuscular junction (del Castillo and Katz 1956; Couteaux 1970). In many synapses of the CNS, particularly those on dendritic spines, the entire presynaptic face of the synapse may constitute an active zone, whereas in other cases, several active zones occur at one synapse (Jahromi and Atwood 1974; Cooper et al. 1995). Synaptic vesicles are preferentially released at the margins of an active zone, as shown in Figure 1F, or within it, in the case of many central synapses. Calcium channels are highly concentrated in the active zone (Fig. 1, D and E). Closely associated with calcium channels in this region are specialized molecules on the vesicles and on the presynaptic membrane, which form a multimolecular complex (core complex; Fig. 1B). The core complex, regulatory proteins, and calcium channel are thought to be mutually linked, and the entire ensemble is sometimes referred to as a secretosome (Bennett 1996). Regulatory molecules and calcium receptors are also present, generating complex molecular interactions (Wu et al. 1999). Only some of the relevant molecules are illustrated in Figure 1B. When this complex is activated by Ca2+, fast exocytosis ensues. Regulatory systems—calcium buffers, calcium sequestration, and extrusion mechanisms—profoundly affect synaptic strength. Postsynaptically, the membrane is specialized for localization of ligand-binding receptors, probably anchored to cytoskeletal components (Van Rossum and Hanisch 1999). Arrays of receptors are often seen in regular alignments in freeze-fracture replicas (Franzini-Armstrong 1976). The structural components are linked
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